In one- and multi-photon fluorescence microscopy, photobleaching of the fluorescent markers under intense light is a limiting factor. A fluorophore under an intense illumination can undergo photobleaching through either unimolecular or bimolecular reaction in which the photon energy and the enviroment play a major rule. Reliable techniques for photobleaching quantum yield (?b) measurements are crucial for fundamental understanding of the mechanisms and pathways involved in bleaching dynamics. On a single molecule level, fluorescence correlation spectroscopy (FCS) is a powerfull technique for diffusion as well as triplet-state characterization. Theoritical analysis has been carried out to extend the use of FCS for photobleaching studies. In those analysis, triplet- and first excited singlet-state are considered as possible photobleaching pathways. Enviromental effects, such as singlet oxygen and external heavy-atom effects, were also discussed.